1. Field of the Invention
The present invention relates to a feed forward amplifier which is employed as a power amplification apparatus in a radio base station for mobile telephones, such as car telephones or portable telephones.
2. Related Arts
Recently, the employment of mobile telephones, such as car telephones or portable telephones, has spread rapidly, and the number of radio base stations established to receive and relay radio waves originating at mobile telephones has also been increased. Such radio base is stations employ incorporated power amplification apparatuses for amplifying received radio waves and for relaying the resultant waves.
FIG. 9 is a block diagram illustrating a feed forward amplifier constituting a power amplification apparatus. In FIG. 9, an electrical signal S1 having a plurality of frequency elements is input to a feed forward amplifier. Subsequently, the electrical signal S1 is amplified by a main amplifier 1. At this time, a distortion element having a frequency differing from the frequency element of the electrical signal S1 appears as noise in an electrical signal S2, which is produced by amplification of the electrical signal S1.
Simultaneously, electrical signal S3 is produced when a delay circuit 2 shifts the phase of one part of the electrical signal S1 180 degrees. And when the thus produced electrical signal S3 and the electric signal S2 carrying the distortion element are synthesized by a directional coupler 6, an electric signal S4 carrying only the distortion element is produced. Thereafter, the electrical signal S4 is amplified to a predetermined level by an auxiliary amplifier 4, and the resultant signal is transmitted to an adder 5.
The adder 5 also receives the electrical signal S3, which was produced when the electrical signal S1 was shifted 180 degrees by the delay circuit 3. Then, when the electrical signal S2 having both a signal element and a distortion element and the electrical signal S4 having only a distortion element are synthesized by the adder 5, the distortion element is canceled out. As a result, an amplified electrical signal S5 which carries no distortion element is output from the adder 5. In this manner, the distortion element is removed from the amplified electrical signal S2.
A power amplification apparatus in which such a feed forward amplifier is employed has a large power consumption, which generally accounts for 60 to 70% of the total power consumed by a radio base station, and the greatest amount of this power is consumed when the radio base station is operating at the maximum transmission power level. But then, even when the transmission output is decreased, there is very little reduction in the power consumption. That is, even when little traffic is being handled by a radio base station and the transmission output is low (e.g., during the night), the radio base station continues to consume a large amount of power. This is true because in a feed forward amplifier, the main amplifier 1 and the auxiliary amplifier 4 are normally A-class amplifiers, so even when the input levels for the electrical signals supplied to these amplifiers are reduced, there is no corresponding reduction in the power consumed by the amplifiers.
In consonance with the above explanation, it is easy to understand that the power consumed by a radio base station greatly affects the operating costs for the station. In addition, contingent upon the need for a large supply power, the size of the power source (battery) installed in a radio base station must be increased, and this militates against there being any reduction in the size of the radio base station. Therefore, the demand has been growing for a method that can optimally control power consumption in accordance with transmission output.
There is one power consumption reduction method, intended for an amplification apparatus for which the transmission output level may become low due to a small traffic volume, for which a power amplification apparatus is constituted by a plurality of feed forward amplifiers, the number of which is reduced when the transmission output volume becomes lower.
In FIG. 10, in a diagram for explaining such a method, is shown the structure of a power amplifier apparatus having four feed forward amplifiers 100. In FIG. 10, a divider 101 for dividing signals input to the feed forward amplifiers 100, and a synthesizer 102 for synthesizing signals output from the feed forward amplifiers 100 are provided before and after the feed forward amplifiers 100. In addition, input switches 103 and output switches 104 are respectively provided between the feed forward amplifiers 100 and the divider 101 and between the feed forward amplifiers 100 and the synthesizer 102. At such time as the transmission output level is at the maximum, all the input switches 103 and the output switches 104 are turned on, and all four of the feed forward amplifiers 100 are set in the operating state. However, when the traffic volume is low, i.e., when the transmission output is but half the maximum, the input and output switches 103 and 104 are rendered conductive/non-conductive, so that only two of the feed forward amplifiers 100 are set in the operating state.
According to this method, however, since a plurality of feed forward amplifiers 100 are provided, a loss in physical deployment flexibility, such as is occasioned by an increase in the size of the power amplification apparatus, occurs. In addition, a power loss in the amplification apparatus, which is attributable to the divider 101, the synthesizer 102 and the input and the output switches 103 and 104, results in an increase in the consumption of power when the transmission output reaches the maximum.